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on commit c085563

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I'm trying to understand your list. Following your approach, we should also use the following names:

not supported by any browser yet
The fact, that they always return undefined in all current browsers only means, that there is no support, but from spec, they all behave equal to other attributes in the current list:

b) equal to not reflecting boolean attributes (i.e.: checked).
- muted (the idl is currently supported by modern browsers, but not the content-attribute to set the initial value. There is also a defaultMuted property)

deprecated/ not often used attributes
My list reducation proposal was heavyly influenced by only adding attributes for back-compat issues. While you try to handle all boolean content-attributes, which have a corresponding idl attribute the same. This means, that also deprecated and less known attributes can be in this list.

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scoped has an idl in IE. The others do not yet have support in any browser. I think we can cross that bridge when they do or we might not be returning consistent values (I'd rather return empty string over undefined for now when those are simply present on the DOM with no value; in a sense, we can rely on the user more for those). As for muted, I'm not sure how to support that without extra code (the audio attribute on a video element sets the muted property for video). The deprecated attributes you mention weren't "supported" in the past and I'd rather push towards keeping them deprecated in the name of progressive enhancement (esp. since there's no backcompat issues there). isMap would require an addition to propFix, so I don't think that's worth it since as you say it is rare. Finally, declare is also deprecated.